Clarify scroll buttons are pseudo-elements, and give order.

scroll-button/README.md

@@ -2,7 +2,7 @@
 
 ## Proposal
 
-Allow the creation of interactive scroll buttons as pseudoclasses, e.g.
+Allow the creation of interactive scroll buttons as pseudo-elements, e.g.
 
 ```css
 .scroller {
@@ -22,6 +22,13 @@ These should be focusable, behaving as a button.
 When activated, a scroll should be performed in the direction by some amount.
 When it is not possible to scroll in that direction, they should be disabled.
 
+The order of the pseudo-elements with respect to the element should be:
+- block direction previous,
+- inline direction previous,
+- the scrolling element,
+- inline direction next,
+- block direction next.
+
 ## Open questions
 
 ### Amount to scroll